The regeneration of adsorbents in many industrial gas adsorption processes is carried out by passing an inert purge gas through the adsorbate-laden adsorbent at an elevated temperature and near-atmospheric pressure. Adsorbed components are thus desorbed and removed from the system in a desorbate-laden effluent stream. In order to conserve purge gas, the desorbate-laden effluent stream can be treated to recover the desorbate, so that the clean gas can be recycled to the regeneration step. U.S. Pat. 4,479,814 discloses a recirculating thermal regeneration system in which hot purge gas is passed through a main adsorber containing spent adsorbent to desorb components adsorbed thereon, cooling the effluent stream and condensing the desorbed component(s). separating the condensed desorbed components, heating the purge stream to the regeneration temperature, passing the purge stream through an auxiliary adsorber, and recirculating the purge gas back to the adsorbent being regenerated. When the main adsorber regeneration is complete, heating is discontinued and the recirculating gas cools the main adsorber in preparation for the next adsorption step. During the cooling step, the auxiliary adsorber serves to remove residual desorbate from the purge gas entering the main adsorber. Adsorbed components removed during the regeneration step can include water or mixtures of C.sub.4 -C.sub.10 petroleum fractions.
A closed-loop process for regenerating an adsorbent used in drying a feed gas stream containing greater than 1 ppm water is disclosed in U.S. Pat. No. 4,484,933. A heated purge gas is passed countercurrently through a main bed of spent adsorbent to desorb water, and the purge effluent is cooled to condense water. The purge gas then is passed through an auxiliary adsorber to remove residual water, heated, and returned to the main adsorbent bed. When the main adsorbent bed is sufficiently regenerated, the purge gas flow through the bed is reversed and cool purge gas passes through the bed to cool the bed in preparation for its next adsorption step. Hot gas from the main bed is passed through the auxiliary adsorber to desorb water, and the effluent gas is then cooled to condense water. The purge gas is then returned to the main adsorbent bed until the bed reaches the desired temperature, and the bed is then returned to adsorption service to dry the feed gas stream. Alternately, water-laden purge gas can be vented and makeup purge gas can be used for cooling the main adsorbent bed.
U.S. Pat. No. 4,536,197 discloses a closed-loop regeneration process in which heat stored during the cooling of a regenerated adsorbent bed is used in the regeneration of another spent adsorbent bed. In the process, a purge gas is heated by passing through a heat storage means, is passed through an auxiliary adsorber and main adsorber which removes a part of the adsorbed components from each adsorber, and is passed through a condenser to remove part of the desorbed components. Heated purge gas from the heat storage means is then passed through the main adsorber for further regeneration, and the auxiliary adsorber is allowed to cool. Finally, purge gas is passed in the opposite direction through the main adsorber, through the heat storage means which stores heat while the main adsorber cools, and through the auxiliary adsorber in which a part of the desorbed components from the main adsorber are adsorbed. Heat losses from the system are made up by an auxiliary heater.
Most inorganic adsorbents including zeolites, silica gels, and aluminas are highly hydrophilic, thus adsorbing water very strongly. In order to use such adsorbents in gas separation processes, adsorbed water must be removed to a high degree in order that the adsorption capacity of these adsorbents is satisfactory for gas separation applications. For certain types of zeolitic adsorbents, damage can occur during regeneration by irreversible chemical reaction between the adsorbent and desorbed components. For example, water-sensitive zeolites such as bivalent ion exchanged type A or X neolites containing water can be damaged during high temperature regeneration by hydrolysis reactions between desorbed water and the zeolite. In order to minimize adsorbent damage in such situations, regeneration must be accomplished at conditions such that these hydrolysis reactions are minimized.